1. Technical Field
The invention relates to a tank withdrawal system for a vehicle tank filled with a frozen liquid. The tank withdrawal system includes a heating system comprising at least one electric heating element serving as a primary heating device and includes a conduit system comprising at least one withdrawal conduit with a withdrawal opening arranged in the vehicle tank. A cold start volume of the frozen substance extending around the withdrawal opening can be melted by the electric heating element. The invention further relates to a method of withdrawing a frozen substance from a vehicle tank, wherein the frozen substance is melted in the region around at least one withdrawal opening and the molten liquid is conveyed out of the vehicle tank through the withdrawal opening.
2. Description of the Related Art
In vehicle tanks for vehicles which supply a consumer with liquid via a conduit system, a problem arises in that the liquids freeze at outside temperatures that are below the freezing point of the liquid. This problem is, for example, known with store tanks for cleansing solutions for windshield and headlight washer systems. In winter, the cleansing solution freezes at low temperatures, so that shortly after a cold start of the vehicle, no liquid cleansing solution for the washer system is available. The addition of antifreeze can lower the freezing point of the cleansing solution only to a limited degree, e.g., to approximately −20° C., so that at temperatures below −20° C. the liquid freezes in the tank and in the conduits of the washer system in spite of the antifreeze.
To reduce nitrogen oxide emissions in the waste gases of internal combustion engines, in particular diesel engines, emission control according to the SCR method (Selective Catalytic Reduction) can be carried out. In this case, an aqueous urea solution serving as a reducing agent is supplied to the exhaust gases of the internal combustion engines. The aqueous urea solution having an urea content of, for example, 32.5 weight percent freezes at a temperature below −11° C.
To avoid freezing of the stored liquids, the store tanks for the cleansing solution of washer systems or for the urea solution are equipped with heating systems which melt off the frozen liquids in the store tanks.
These heating systems are conventionally integrated in the withdrawal unit and melt off at least a cold start volume of the liquid to be withdrawn that is present around the withdrawal opening in the tank during a cold start of the vehicle.
A heating system to be used in fuel tanks to avoid precipitation of the fuel at low temperatures in diesel tanks is known, for example, from U.S. Pat. No. 4,656,979. The heating system described therein comprises a conduit system through which, for example, the coolant of the internal combustion engine is conveyed, of which the convected heat heats the fuel. The conduit system comprises two conduits extending along the tank withdrawal conduit which is followed by a conduit section at a right angle extending in parallel to the outside wall of the tank.
A heated liquid tank for a motor vehicle has a heating device that uses energy from an electrical source of energy is described in WO 03/093080 A1. The battery of the vehicle, for example, functions as electrical source of energy. The heating device, for example, is a thick film heater with an element, such as a resistor or as helical electrical resistance wire.
A heated tank withdrawal system is known from EP 1 582 732 B1. It comprises an immersed body provided with a withdrawal conduit and a heat exchanger. The conduits of the heat exchanger, through which, for example, heated coolant flows extend along the withdrawal conduit and into a heat exchanger extension that extends away from the immersed body into the interior of the tank. The tank withdrawal system can comprise an additional electric heating which additionally heats the withdrawal conduit when the coolant is still cold.
A device described in DE 10 2006 027 487 A1 utilizes the convection effect in that a sheet-like aluminum body provided with heating elements and with convection holes at the edge is arranged near the bottom of the tank. Around the convection holes, a convection flow is formed by which the heat in the already thawed medium is continuously transported upwards. The medium cools further at the top and sinks again downwards to the heating.
The disadvantage of the conventionally used tank withdrawal systems is that a relatively high amount of energy is required to melt a sufficient amount of frozen reducing agent in the tank to the operation of the SCR catalyst. Thermal energy is only transmitted to the liquid across a limited area. To melt frozen reducing agent in the tank at a greater distance from the heating unit, the reducing agent present near the heating unit must be heated to a temperature above its freezing point. This is the only way for a convection flow or heat conduction to reach more remote regions of the tank through the ice and to melt off the frozen reducing agent within an appropriate period of time.
That means, to completely melt off the tank contents, a high temperature gradient is required which heats the reducing agent altogether more than necessary. Nevertheless, the temperature of the heating unit must not be arbitrarily high, as vapor forms near the heating unit when the boiling point of the liquid to be melted or of the component of the liquid having the lowest boiling point is exceeded. The vapor bubbles formed have a thermally insulating effect and reduce the efficiency of the heating device. When aqueous urea solutions are heated to temperatures of more than 60° C., the urea may decompose, so that this temperature must not be exceeded at the heat transfer surfaces.
An aggravating factor is that melting begins directly at the heating surface, so that after the molten liquid has drained off, a layer of air can be formed around the heating unit. This layer of air also has a thermally insulating effect and can thus drastically reduce the efficiency of the heating device.